This invention relates to a method for controlling, in a microwave oven, the feeding of microwaves to the oven cavity, the oven comprising a microwave radiation source and a control unit for controlling the microwave feed, means being arranged in the cavity for bringing about a periodically-varying microwave exposure of the food or dish during heating, and a desired power level below full power of the fed microwaves being produced by periodic activation of the microwave radiation source during a control cycle that is part of a sequence of control cycles. The periodically-varying microwave exposure may be due to a periodic movement of the food, e.g. produced by a rotary bottom plate carrying the food or dish, or be due to a periodic variation of the microwave-field distribution in the cavity, e.g. produced by a rotary field agitator or aerial which usually is disposed at the ceiling or bottom of the cavity adjacent to where the microwaves are fed to the cavity. Combinations of these effects may also be used.
The invention further concerns a microwave oven having a control unit operating in accordance with the inventive method.
A general problem in microwave ovens is to achieve a microwave distribution in the cavity that optimises the uniformity of the heating of the food or dish placed in the cavity. If the microwave distribution is uneven, there will be "hot" and "less hot" zones in the cavity, and the parts of food located in these different zones will be heated to a different extent. This problem is aggravated by the fact that the microwave properties, the volume and the weight of the food, as well as the receptacle containing the food, affect the microwave distribution in the cavity. Even though it is possible to achieve a good microwave distribution in some specific cases of operation by suitable dimensioning of the cavity and its microwave-feed system, the problem still remains, the number of conceivable cases being virtually unlimited. This results in non-uniform heating of the food or dish.
A common way of improving the heating uniformity is to introduce a so-called rotary bottom plate in the oven cavity. The food or dish is then placed on the bottom plate, which rotates during the heating procedure. Since the food is thus rotated, its different parts will pass both the "hot" and the "less hot" zones during the heating procedure. Such bottom plates are, inter alia, used in the applicant's microwave ovens of type designations VIP20 and VIP 27. Alternatively, the heating uniformity can be improved by manipulating the microwave-field distribution in the cavity with the aid of a rotary field agitator or aerial, which may be disposed in the ceiling or bottom of the cavity adjacent to where the microwaves are fed to the cavity, so as to "agitate" or spread the microwaves. SE Patent 8006994-1 teaches such an oven construction. Thus, the microwave-field distribution in the cavity is varied periodically, as a function of the speed of rotation of the bottom plate or of the field agitator or aerial.
Different heating procedures require an adjustment of the power level of the microwaves fed to the cavity. During one and the same procedure, different power levels may be used during different periods of the procedure. A common way of achieving different power levels of the microwaves fed to the cavity is to divide the cooking procedure into control cycles and activate the microwave radiation source (normally a magnetron) of the oven periodically during these cycles. The power level is then determined by the average power of each cycle. SE Patent 8800323-1 teaches a microwave oven with such power control. This method of power control will be described in more detail below.
However, the problem of non-uniform heating can be aggravated by using the above power control in a microwave oven having a rotary bottom plate or a field agitator or aerial. In general, this is due to an interaction between the revolution time and the duration of the control cycle (normally in the same order), which may entail that a certain part of the food or dish periodically is found in the same part of the cavity volume during that interval of the control cycle when microwaves are fed to the cavity.
A rotary bottom plate is commonly operated with the aid of a synchronous motor imparting a constant speed of rotation, typically of about 5-6 revolutions/min, to the bottom plate, which means that the constant revolution time typically is 10-12 sec. In the above power control, the duration of the control cycle usually is 15-30 sec, i.e. substantially of the same order as the revolution time mentioned above.
An obvious solution to this problem would be to use a much shorter duration of the control cycle, so that the microwave radiation source, which normally is a magnetron with an output power in the order of 1 kW, would have to be switched on and off at a fairly high frequency. There are, however, many factors that tell against such a solution: the wear of the magnetron component increases and drastically reduces its service life; the mains operators in different countries do not allow too-rapid switching of the current power level, since this has a considerable interfering effect on the mains; and the food or dishes involved have limited thermal conductivity, which means that a certain time is needed to distribute or even out the supplied microwave energy in order to achieve good heating results.